Wanna be engine builders
 

Here's a good article on rings and the bore. I've always stated that a round straight bore is important for not only ring seal but overall engine life.
http://www.enginebuildermag.com/2011...ct-horsepower/

Very interesting article! It leads me to question Kohlers' ring end-gap specification of .010 - .020 inches for the K321 engine!
From the article: "End Gaps McKnight says the optimal end gap for a top compression ring is when you achieve the smallest gap without butting the ends of the ring together when the rings get hot. “A good starting point for the top compression ring is .0045? per inch of bore diameter for a naturally aspirated engine,"
The K-321 engine approximate piston diameter = 3.5 inches, 3.5X.0045= .016 inches end gap.
I gapped my rings at .011 -.012 on my recent rebuild job, Is this end gap too tight for an air cooled engine? Can't imagine a water cooled engine needing more end gap.

You are in Kohler's spec do not worry about it. Your fine.

the article wasn't meant to reengineer the Kohler manual. for a stock engine always follow the given specs. We use allot of automotive stuff in our pulling engines forged pistons, moly rings, special cyl coatings this is where you have to pay special attention to detail. for probably 95% of the people on this forum there's no need to know any of the stuff in that article. My point was how critical bore is in ring life and ring seal and how it can affect an engine. I can't say it enough

Thanks Austin,
Sounds good to me! But if I do another rebuild, I just may look into end gap recommendations of other cast iron air cooled engine manufacturers...

Seeing as how many engines you have rebuilt, how often have you had to have a cylinder redone because of faulty machining? How closely do you check the bore on the cylinders you get back from the machine shop? Just curious, this engine tech is very interesting.

I always check what comes back from the machine shop until I trust them. The current shop I use I don't have to check up on. They are always spot on.

you have to check them. even good machine shops make errors. for the time it takes to check clearance's vs. rebuilding an engine because of an error it would be foolish not to. you really don't have to spend allot on fancy stuff either. you really don't need micrometers , bore gages ect simply checking piston side clearance, ring end gap and plastigage the rod to crank . a set of long feeler gauges and plastigage is all that's needed to catch a machining error at a cost of $15 for both . even crankshaft and cam endplay can be checked with feeler gages if you don't have a dial indicator. hard to get by without a torque wrench. think about it this way; the guy putting the engine together is the last chance you have to fix something that could go horribly wrong. you are the quality check. over the years I've noticed allot of guys blaming the machine shop for their engine failure because this wasn't right or that wasn't right. bottom line you have to check stuff as you assemble.
its difficult for people that want to build their one engine so the can say I did it. I get that, small engines are relatively simple , cheap and easy work on. you mess it up you're only out a few hundred dollars. but a few hundred dollars still buys allot of donuts, Coke or beer.

I have all my cylinders rough bored. I finish hone the bore and fit the piston myself. so I don't have issues with that. I did have one block bored where they went an 1/8 in to big. they told me about it and made it right

Here's a good story about how we find out what works and what don't. We were playing on the dyno one day. I did my research and I decided I was going to try synthetic 0 weight oil along with a few other things. we broke the motor in and fine tuned it. put 0 weight oil in and made a run which looked promising. made a change in jetting. made a second run. lost allot of power. jetted back to where it was and made another run and the death breath started huffing out the breather. bore was good. rod bearing wasn't. cut the crank , new bearing and back together for the next weekend. that was around 13 years ago. no more 0 weight oil here. but there are gains in light weight motor oil. you just can't get greedy and go overboard

Dam. Must have been a Monday job!! :beer2: